1. Field of the Invention
The present invention relates to a liquid crystal display device with a touch panel and a terminal device incorporating such a liquid crystal display device.
2. Description of the Related Art
Recently, liquid crystal display devices have widely been used in terminal devices including large terminal devices such as display monitors, television sets, etc., medium terminal devices such as notebook personal computers, ATMs (Automated Teller Machines), etc., and small terminal devices such as personal television sets, PDAs (Personal Digital Assistants), cellular phones, portable game machines, etc. In addition, liquid crystal display devices combined with a touch panel, which function not only as a display means but also a simply operable input means, are also widely used in medium and small terminal devices.
In particular, many small portable terminals such as PDAs incorporate a resistance-film touch panel which is low in cost and size. However, the resistance-film touch panel has been problematic in that it lowers the contrast ratio of the liquid crystal display device when used in outside light. A method for solving the problem is disclosed in JP-A No. 10-048625 (Patent Document 1).
FIG. 1 of the accompanying drawings is a schematic cross-sectional view showing a structure of a liquid crystal display device with a touch panel disclosed in Patent Document 1. As shown in FIG. 1, the liquid crystal display device with the touch panel comprises liquid crystal display device 107 and a resistance-film touch panel superposed on liquid crystal display device 107. The touch panel includes spacer 103, and two transparent substrates 101 disposed in confronting relation to each other across spacer 103. Transparent electrode 102 is disposed on one surface of each of transparent substrates 101. First quarter-wave plate 104 is interposed between liquid crystal display device 107 and the touch panel. A circular polarizer comprising polarizer 106 and second quarter-wave plate 105 is disposed on the touch panel.
The contrast ratio of a liquid crystal display device refers to a value produced as a quotient when the luminance of a fully white image is divided by the luminance of a fully black image. An air layer is present between a pair of confronting transparent electrodes of a resistance-film touch panel. Therefore, the interfaces between the transparent electrodes and the air layer produce relatively large surface reflections. Even while a fully black image is being displayed, outside light applied to the touch panel is reflected by the interfaces between the transparent electrodes and the air layer and returns to the observer. As a result, the luminance of the fully black image increases, and the contrast ratio decreases.
With the liquid crystal display device with the touch panel disclosed in Patent Document 1, before outside light is applied to the touch panel, it is converted into circularly polarized light by the circular polarizer disposed on the touch panel, and the circularly polarized light is reflected by the interface between transparent electrode 102 and the air layer. When the circularly polarized light is reflected by the interface, it is converted into circularly polarized light which is in a direction opposite to the direction of the applied circularly polarized light. Therefore, the reflected light is absorbed by the circular polarizer. As a consequence, the contrast ratio is prevented from being lowered by the outside light reflected by the interface between transparent electrode 102 and the air layer.
Light emitted from general liquid crystal display devices is linearly polarized light, and light emitted from liquid crystal display device 107 disclosed in Patent Document 1 is also linearly polarized light. The liquid crystal display device with the touch panel disclosed in Patent Document 1 includes first quarter-wave plate 104 for converting the light (linearly polarized light) emitted from liquid crystal display device 107 into circularly polarized light. Therefore, the light emitted from liquid crystal display device 107 is not absorbed by the circular polarizer, but passes efficiently through the circular polarizer.
For further suppressing the reduction of the contrast ratio due to the reflection by the touch panel, the circular polarizer should preferably function as a circular polarizer for visible light in various wavelength bands, i.e., a wide-band circular polarizer. Generally, two methods, described below, may be used to provide a wide-band circular polarizer.
According to one of the methods, a circular polarizer comprises a single polarizer and two or more phase retarders. Specifically, a circular polarizer comprises a polarizer, a half-wave plate, and a quarter-wave plate. In the pre-sent description, the method wherein a circular polarizer comprises a single polarizer and two or more phase retarders is referred to as “double phase retarder design”.
According to the other method, a circular polarizer comprises a single polarizer and a phase retarder whose retardation value is represented by a quarter-wavelength in a wide visible range (hereinafter referred to as “reverse dispersion quarter-wave plate”). In the present description, this method is referred to as “single phase retarder design”.
Liquid crystal display devices are generally classified into transmissive, reflective, and semitransmissive types. The semitransmissive liquid crystal display device employs transmitted light and reflected light. The reflective liquid crystal display device employs is of a smaller power consumption requirement as it uses outside light for display, but has poorer display performances such as the contrast ratio, etc. than the semitransmissive liquid crystal display device. At present, therefore, the transmissive and semitransmissive liquid crystal display devices are mainly used in the art. The transmissive and semitransmissive liquid crystal display devices include a light source disposed behind the liquid crystal display panel and display an image using light emitted from the light source. Particularly, medium and small liquid crystal display devices that are carried by the users and used in various situations widely comprise semitransmissive liquid crystal display devices which use a reflective display mode in bright environments and a transmissive display mode in dark environments.
Heretofore, liquid crystal display panels for use in the semitransmissive liquid crystal display devices have employed an ECB (Electrically Controlled Birefringence) mode and a multidomain vertical alignment mode for higher image quality and wide field-of-view angle. Attempts for applying a lateral electric field mode which has a wider field-of-view angle in principle to a semitransmissive liquid crystal display device is disclosed in JP-A No. 2005-106967 (Patent Document 2).
FIG. 2 of the accompanying drawings is a schematic exploded perspective view showing a structure of the semitransmissive liquid crystal display device disclosed in Patent Document 2. In FIG. 2, an XYZ orthogonal coordinate system is defined as follows: The direction from liquid crystal layer 206a toward polarizer 202 is defined as a +Z-axis direction, and the opposite direction as a −Z-axis direction. Directions extending parallel to the sheet of FIG. 2 and perpendicularly to the Z-axis are defined as X-axis directions. The rightward direction on the sheet is defined as a +X-axis direction, and the leftward direction as a −X-axis direction. A +Y-axis direction is a direction in a right-handed coordinate system. Specifically, when the thumb of the right hand is oriented in the +X-axis direction and the index finger in the +Y-axis direction, the middle finger points to the +Z-axis direction.
The semitransmissive liquid crystal display device shown in FIG. 2 comprises a polarizer, a phase retarder, a liquid crystal layer, and a reflector. In the present description, the placement angle of the phase retarder is expressed as the angle formed between the retardation axis (the retarder axis) and the X-axis. The counterclockwise direction as viewed from the +Z-axis direction is defined as positive. The placement angle of the polarizer is expressed as the angle formed between the absorption axis of the polarizer and the X-axis. The placement angle of the liquid crystal layer that is horizontally oriented is expressed as the angle formed between the orientation axis of the liquid crystal layer and the X-axis when no voltage is applied to the liquid crystal layer.
An Nz coefficient and the angle dependency of a retardation used in the present description will be described below. The Nz coefficient is expressed as:Nz=(nx−nz)/(nx−ny)  (1)where nx represents the refractive index of a birefringence medium (phase retarder or the like) in a direction (retarder axis) to maximize the refractive index within the film plane thereof, ny the refractive index in an in-plane direction perpendicular to the retarder axis, and the nz the refractive index in a thicknesswise direction.
As indicated by the equation (2) shown below, a retardation Re(0) with respect to light parallel to the normal line of the retarder is determined by the difference between the refractive indexes nx, ny in the in-plane main axis directions and the thickness d of the phase retarder.Re(0)=(nx−ny)×d  (2)
A retardation with respect to light inclined to the normal line of the retarder is affected by the refractive index nz in the thicknesswise main axis direction and the optical path length that has increased because the optical axis is inclined to the normal line. In the present description, the retardation with respect to light parallel to the normal line of the retarder will be referred to as “retardation” or “Re(0)” and the retardation with respect to light inclined to the normal line of the retarder will be referred to as “oblique retardation” or “Re(θ)”. In particular, the retardation with respect to applied light whose optical axis is inclined to the retarder axis will be referred to as “Rex(θ)”, the retardation with respect to applied light whose optical axis is inclined in the in-plane direction perpendicular to the retarder axis will be referred to as “Rey(θ)”.
For phase retarders for use in liquid crystal display devices, it is known in the art that Re(0), Rex(θ), and Rey(θ) are related to each other depending on the Nz coefficient, as follows: For a phase retarder with Nz=1, the relationship nz=ny is satisfied according to the equation (1). Therefore, when the index ellipsoid is observed from the direction of normal line of the substrate, it is shaped like a rugby ball whose length is greatest in the direction of the retarder axis. At this time, the relationship Re(0)>Rex(θ), Re(0)<Rey(θ) is satisfied.
For a phase retarder with Nz=0, the relationship nz=nx is satisfied according to the equation (1). Therefore, the index ellipsoid is of a shape whose length is greatest in the direction of the retarder axis and the thicknesswise direction. At this time, the relationship Re(0)<Rex(θ), Re(0)>Rey(θ) is satisfied.
It is known that when the phase retarder with Nz=1 and the phase retarder with Nz=0, whose Re(0) are equal to each other, are disposed such that their retarder axes are perpendicular to each other, the phase retarder with Nz=1 cancels out the refractive index anisotropy of the phase retarder with Nz=0.
For a phase retarder with Nz=0.5, the relationship nz=(nx+ny)/2 is satisfied. Therefore, the index ellipsoid is of a shape which is intermediate between the shape of the phase retarder with Nz=1 and the shape of the phase retarder with Nz=0. It is known that Re(0)≈Re(θ) in a wide range of θ (see SID1992, DIGEST, pages 397-400).
Referring back to FIG. 2, the liquid crystal layer is divided into liquid crystal layer 206a corresponding to a transmissive display region and liquid crystal layer 206b corresponding to a reflective display region. If the transmissive display region of the liquid crystal display device is viewed from the display screen, then it comprises polarizer 202, half-wave plate 205 (Nz=1), liquid crystal layer 206a, phase retarder 204 (Nz=0), half-wave plate 203 (Nz=0), and polarizer 201 which are stacked together in the order named. If the reflective display region of the liquid crystal display device is viewed from the display screen, then it comprises polarizer 202, half-wave plate 205 (Nz=1), liquid crystal layer 206a, and reflector 207 which are stacked together in the order named.
The liquid crystal layer is horizontally oriented. Liquid crystal layer 206b has a retardation represented by a quarter wavelength. Liquid crystal layer 206a has a retardation which is slightly smaller than twice the retardation of the reflective display region. Specifically, the retardation of liquid crystal layer 206a is in the range from 1.7 to 1.9 times the retardation of the reflective display region. If one wavelength is 550 nm, then the retardation of liquid crystal layer 206a of the transmissive display region is in the range from 233.8 nm to 261.2 nm.
The placement angle of polarizer 202 is 90 degrees. The placement angle of half-wave plate 205 is 15 degrees. The placement angle of the liquid crystal layer is 75 degrees. The placement angle of phase retarder 204 is 165 degrees. The placement angle of half-wave plate 203 is 105 degrees. The placement angle of polarizer 201 is 0 degree. Polarizer 202, half-wave plate 205, and liquid crystal layer 206b of the reflective display region jointly make up a wide-band circular polarizer.
The above optical configuration of the liquid crystal display device shown in FIG. 2 makes it possible to equalize voltage-controlled operation of liquid crystal layer 206a of the transmissive display region and liquid crystal layer 206b of the reflective display region. Specifically, when no voltage is applied to the liquid crystal layer, both the transmissive display region and the reflective display region are dark.
However, if the liquid crystal display device with the touch panel disclosed in Patent Document 1 and the lateral-field semitransmissive liquid crystal display device disclosed in Patent Document 2 are combined with each other, then the types and numbers of the phase retarders and wave plates are increased, resulting in an increase in the cost. The thickness of the liquid crystal display device is also increased. Furthermore since the types of paired phase retarders are different from each other, the contrast ratio of the transmissive display may possibly be reduced. These shortcomings will be described below.
As shown in FIG. 2, the lateral-field semitransmissive liquid crystal display device disclosed in Patent Document 2 requires two polarizers and three phase retarders of different types (half-wave plate 205 (Nz=1), phase retarder 204 (Nz=0), and half-wave plate 203 (Nz=0)).
Therefore, if the circular polarizer of the liquid crystal display device with the touch panel shown in FIG. 1 comprises a circular polarizer of the single phase retarder design and liquid crystal display device 107 comprises the semitransmissive liquid crystal display device disclosed in Patent Document 2, then the resultant liquid crystal display device requires three polarizers and five phase retarders of four types. Specifically, it requires three polarizers including polarizer 106 shown in FIG. 1 and polarizers 202, 201 shown in FIG. 2, and five phase retarders including quarter-wave plates 104, 105 shown in FIG. 1, half-wave plate 205 (Nz=1), phase retarder 204 (Nz=0), and half-wave plate 203 (Nz=0) shown in FIG. 2.
If the circular polarizer of the liquid crystal display device with the touch panel shown in FIG. 1 comprises a circular polarizer of the double phase retarder design, then the number of required phase retarders further increases. Specifically, the resultant liquid crystal display device requires three polarizers and seven phase retarders of four types.
In order to prevent the contrast ratio of the transmissive display from being lowered, half-wave plate 203 (Nz=0) and half-wave plate 205 (Nz=1) shown in FIG. 2 need to cancel out their respective retardations. Usually, the phase retarder with Nz=0 and the phase retarder with Nz=1 are made of different materials. For example, polystyrene is known as the material of the phase retarder with Nz=0, and polycarbonate is known as the material of the phase retarder with Nz=1. Therefore, the refractive indexes of half-wave plate 203 and half-wave plate 205 have different wavelength dispersions, and their retardations do not sufficiently cancel out each other. If half-wave plate 203 and half-wave plate 205 have their Re(0) canceled out insufficiently, then the contrast ratio of the transmissive display is lowered.
As described above, a number of optical films have been required to equalize the operation of the reflective and transmissive display regions of the semitransmissive liquid crystal display device while suppressing the reduction of the contrast ratio due to the reflection by the touch panel.